Hold-current-limiter

In many applications, a solenoid driver must first briefly supply a large amount of pull-in cnrrent, which quickly actuates the solenoid. Thereafter, the driver must supply a much lower holding cnrrent to avoid burning the solenoid out. To avoid using the customary, cumbersome, large capacitors or power-wasting resistors, you can use the switch technique.

As long as the input to the circuit is low, diode Dl holds Ql off; a low input also prevents the op-amp circuit from oscillating. When the input reaches 24 V, Ql switches on and pulls in the solenoid. Concurrently, D4 is back-biased, and Cl begins charging up. When Cl charges up, the op-amp circuit begins to oscillate, switching Ql on and off. The time constant defined by Rl and Cl determines the length of the period during which the solenoid receives full power. R3 and C2 set the oscillator"s frequency, and R2 sets the oscillator"s duty cycle. The hold cnrrent is directly proportional to the duty cycle. For the components shown, the full-power period is 300 ms, the oscillator"s frequency is 3kHz, and its duty cycle is 50%.